As illustrated in U.S. Pat. No. 3,924,604 issued to Donald E. Anderson, on Dec. 9, 1975, it is known to provide a solar energy receiver on the top of a mast, below which is provided a mirror field which includes a number of mirrors which redirect incoming solar radiation and focus it onto the solar receiver. In this patent ingested air is pumped from the top of the tower to the bottom of the tower and thence to a boiler which provides a mechanism for transferring the focused solar energy into usable energy by virtue of fluid heated in the boiler.
One of the outstanding problems in this type system is the necessity of pumping the air from the receiver down through the tower to a subsurface boiler, which pumping energy is considerable.
In the Anderson patent it is further noted that louvers are heated by focused solar radiation. The heating of louvers is, to a certain extent, inefficient when coupled with the amount of flow necessary to remove the heat from the louvers and then transfer this energy down to the base of the tower which may be on the order of several hundred feet high.
In the subject invention the heat exchanger, rather than being located at some distance from the point at which solar radiation is focused, is placed above a porous high-temperature absorbing material at or near the focus of the solar radiation. The receiver in one embodiment is an open-ended device which includes a cavity having an opening and a porous body member or element positioned at or near the opening. Heat from the porous body is transmitted to a conventional steam-type heat exchanger which is positioned above and adjacent to the porous body primarily by virtue of the convectively-rising hot air. In one embodiment, temperature control is obtained by regulation of an exhaust fan, while in another embodiment control is obtained by a variable orifice at the top of the receiver. When an exhaust fan is used, pumping energy expended by the exhaust fan utilized is minimal because of the convectively rising hot air and the particular position of the heat exchanger above the heated porous body.
Other types of steam generating plants employing a receiver mounted on a tower are exemplified by U.S. Pat. No. 3,927,659 issued to Floyd A. Blake et al on Dec. 23, 1975, in which a closed receiver having a window employs heat exchanger tubes within the body of the receiver to convert the solar energy into steam. Here no porous member is employed. However, it is notable that in this patent at column 1, lines 38 through 43, it is said that "convection problems have been minimized to some extent by locating the boiler super heater units above the reflection mirror field with the cavity or conversion chamber at the bottom of the unit and facing vertically the mirror field, and while this trapped, to some extent, hot air from convection in the chamber, this adversely limits the physical plant and results only in minimizing to some degree convection losses." Thus, as exemplified by this patent, the use of convection as the means of transferring solar energy was avoided and was generally thought to be a serious defect.
Contrary to the teaching of this patent, it has been found that by the utilization of the open ended concept, along with a heat exchanger above a porous body, transfer of energy be convection is, in fact, more efficient and, therefore, rather than being avoided is, in fact, sought. The efficiency increase comes from minimizing the pumping energy which is utilized in either one of the two above-mentioned patents and by the proximity of the boiler to the heated porous body.
One purpose of providing the porous solar absorber is to provide a controlled temperature fluid (air) as the heat exchange medium. This prevents high temperature areas from occurring on the heat exchanger due to solar energy distribution non-uniformities if the heat exchanger were to be heated directly by the focused solar radiation. It should be noted that solar radiation distributions have been measured to vary by as much as 10:1, which could burn out a directly illuminated heat exchanger. Further, a loss of coolant (water) with a direct solar impingement heat exchanger would result in very high temperature and a burnout or meltdown. In contrast, in the embodiment presented, there is a maximum controlled upper limit to the air temperature, and this upper limit is provided by the solar absorber. This, in turn, prevents heat exchanger failure or requirement for repair.
It is, therefore, the object of this invention to provide an efficient solar receiver in which pumping energy is minimized;
It is another object of this invention to provide constant temperature solar heated air to a heat exchanger regardless of solar density variations, thereby to prevent damage to the heat exchanger. PA0 It is another object of this invention to provide the combination of an openended solar receiver utilizing a porous member and a closely spaced heat exchanger which receives the majority of its energy through flow caused by convection; PA0 It is still further object of this invention to locate a heat exchanger above a porous body heated by focused solar radiation in which the energy from the focused solar radiation is transferred to the heat exchanger via convective currents. These and other objects will be better understood in connection with the following specifications when taken in conjunction with the following drawings.